1. Field of the Invention
The present invention relates to a ballast unit and also to a method and a device for disinfecting water.
2. Description of Related Art
A generic ballast unit is disclosed in DE 196 37 906 A1. In this so-called electronic ballast unit, the connected radiator is operated during operation at a frequency of about 20 kHz to 50 kHz, whereas the conventional ballast units operate at the mains frequency of 50 Hz. As also in the case of conventional ballast units, two connecting leads are routed to each heating coil of the connected gas-discharge lamp. Via said leads, a heating voltage is first applied to the heating coils and brings about heating of the coil. As soon as the temperature is high enough to make it possible for electrons to escape from the surface, the gas discharge is ignited by a high-voltage pulse. The heating voltage is switched off and the operating voltage of the gas-discharge lamp is applied to a connecting lead of each heating coil. The switching between heating voltage and operating voltage takes place in the generic ballast unit with semiconductors. During operation, voltage is applied to only one conductor per coil so that the respective other conductor is unconnected. The power of the connected UV radiator is automatically controlled by a pulse-width modulation.
Because one coil conductor is used in each case only for heating and is an open-circuit conductor during operation, the reactive effect on the heating circuit due to reflections in the cable is very great. If a cable length of 8 m between the lamp and the ballast unit is exceeded, the oscillatory behavior at the heating circuit in combination with the high frequency of the operating voltage of about 20 kHz to 50 kHz is so severe that it may result in the destruction of the circuit. Furthermore, the oscillatory processes are transformed onto the primary side of the push-pull output stage used in the known ballast unit. During the switching of the semiconductor switches, high-frequency oscillatory processes occur at the instant of switching that result in a severe loading of the semiconductor switches.
For this reason, the distances between a control unit containing the ballast units and the UV radiator arrangement operated therewith in a wastewater or drinking-water disinfecting system have hitherto not been able to exceed about eight meters, which is regarded as constructionally disadvantageous, in particular in the case of extensive systems.
WO 98/24277 discloses a quick-start circuit for fluorescent lamps that can be combined both with conventional chokes and with the electronic ballast units common in this technical field. Both techniques require a series oscillatory circuit having a choke in the lamp supply line, which at the same time brings about the current limitation. In such circuits, the supply current applied to the lamps is essentially sinusoidal. These ballast units and quick starters are always disposed in the immediate vicinity of the luminaires. Problems with the conductor length between the ballast unit and the luminaire are irrelevant in this case.
To reduce the stress on the electrodes, they are short-circuited in the case of the known quick-start device immediately prior to ignition and during operation and the feed conductors are connected in parallel. This parallel connection takes place between the quick-start device and the electrodes. The two lamp feed conductors emerging from the ballast unit are themselves not connected in parallel between the ballast unit and the quick-start device. This circuit does not solve the problem of large conductor lengths in electronic ballast units according to DE 19637906 that operate with high-frequency supply voltages having great edge steepness. The ballast units do not have a choke in the feed conductor and can therefore generate, for example, a rectangular current variation.